In a radiotelephony system, the mobile terminals must be capable of receiving signaling while a call is set up on a traffic channel, so as to exchange information relating to the services offered to the terminals or to the management of the radio links.
In frequency-duplexing cellular systems such as GSM, signaling channels called SACCH are associated with the traffic channels. They use the same carrier frequencies as the traffic channels, via a time-division multiplexing mechanism. These signaling channels, like their associated traffic channels, are in full duplex mode.
This is not well suited to the needs of professional radiocommunication systems, which frequently operate in alternate mode, and for which the group communications play a very important role. In such a system, a known solution consists in making provision for a pair of control frequencies which are common to all the terminals attended to by a base station, and over to which the communicating mobile terminals periodically transfer for the needs of the associated signaling channels. This solution is not very flexible. Furthermore, it demands synchronism between the traffic frames and the control frames, this having the drawback of prohibiting the monitoring and presynchronization mechanisms which allow better management of radio resources, in particular through the effective implementation of intercell handovers.
Patent Application EP-A-0 896 443 describes a system of radiocommunication with mobile terminals exhibiting the feature of offering time-division multiplexing services with various degrees of protection related to the possible use of modulation coded on the carrier. For a given throughput, offered for the execution of the service, the number of timeslots allotted to the service is related to the coding or to the absence of coding of the modulation, and/or to the rate of the coding applied. In a particular execution of this system, one and the same service can be offered in a first mode on a half-channel with noncoded modulation, or in a second mode on a full channel with modulation coded by a code of rate ½.
Patent application EP-A-0 677 930 describes a frequency-division multiplexing radiocommunication system in which a carrier frequency supports a radio signal organized as multiframes comprising traffic frames and signaling frames, uplink or downlink, reserved for the mobile stations in the transmission mode.
Patent Application EP-A-0 644 702 describes a time-division multiplexing radiocommunication system in which time intervals of frames shifted between the uplink and the downlink support logical signaling channels, uplink or downlink, for mobile stations in the transmission mode or reception mode.
The aim of the present invention is to propose an organization of associated signaling channels which properly meets the needs of professional radiocommunication systems.